Rising energy prices, the growing dependency on foreign oil, and environmental concerns have increased public and government interest in renewable sources of electricity generation. Photovoltaic devices, also known as solar cells, generate electrical power from ambient light. The solar power market has continuously grown in popularity and the ability to create high-efficiency solar cells is a key strategy to meeting growing world energy needs. Today's photovoltaic systems are predominantly based on the use of crystalline silicon, thin-film and concentrator photovoltaic technologies.
Crystalline silicon technologies can be differentiated into mono- or single-crystalline and poly- or multi-crystalline technologies. It has been estimated that crystalline silicon technologies represent almost 70 percent of the North American residential solar cell market. Mono-crystalline cells contain a uniform structure achieved by highly controlled manufacturing processes which require large amounts of the expensive silicon. Multi-crystalline cells contain small, individual crystals oriented in different directions. These cells use lesser amounts of the expensive silicon compared to mono-crystalline cells, but achieve lower efficiencies. Ribbon technologies, which incorporate a variation on the multi-crystalline production process, use fewer types of raw materials but also have lower energy conversion efficiency.
Thin-film technologies have lower efficiencies than crystalline silicon cells but permit direct application to a surface that can be either glass or plastic. Thin-film technology reduces end product costs because it allows for smaller amounts of semiconductor material to be used, can be manufactured by a continuous process, and results in a product that is less likely to be damaged during transportation. Thin-film technologies can also allow for applications on curved surfaces. Thin-film technologies have several drawbacks however. Amorphous silicon thin-film technologies use expensive silicon and have relatively low energy conversion efficiencies. The main drawback of cadmium telluride thin-film technologies is the toxicity of cadmium. The main disadvantages of copper indium diselenide and copper indium gallium diselenide technologies are the complexity involved in the manufacturing process, as defects easily form, and safety issues concerning the manufacturing process, which involves the extremely toxic gas hydrogen selenide.
Concentrator photovoltaic technologies provide high efficiencies through the use of concentrating optics which bundle arrays. Concentrator photovoltaics increase power output while reducing the number or size of cells needed. The main drawback to this technology is the requirement of expensive tracking systems. Concentrator photovoltaics can only use direct sunlight and therefore require a means to follow the movement of sunlight. Concentrator technologies are based on either crystalline silicon or gallium arsenide (GaAs). While silicon is very expensive, GaAs is fragile, a known carcinogen and is even more expensive than silicon.
As discussed above, current solar cell technologies have several drawbacks. Most existing technologies use expensive raw materials. Some traditional solar cells incorporate glass or plastics which makes the cells heavy, potentially dangerous, and expensive to ship. Some cells are expensive to install because they must be specially mounted or integrated with expensive tracking mechanisms. Other technologies use dangerous materials in the manufacturing process or final product. Also, up to 70% of the expensive silicon is wasted during some silicon solar cell manufacturing methods. Another drawback of some traditional silicon based photovoltaics is that they are rigid. While it is possible to incorporate these cells into fabric or other flexible material, the cells themselves remain solid. The electronics required for traditional cells adds further complexity to using them as anything other than standalone add-ons for devices or buildings. Lastly, existing silicon and other solar cell technologies may be reaching their limit in terms of cost to efficiency ratios.
Nanotechnology is currently enabling the production of organic photovoltaics (OPVs) to help meet the world energy demand and overcome the disadvantages associated with traditional silicon based photovoltaics. Organic photovoltaics are composed of layers of semiconducting organic materials (polymers or oligomers) that absorb photons from the solar spectrum. In OPVs, solar radiation promotes the photoactive semiconducting organic materials in the photoactive layer to an excited state. This excited state is referred to as an exciton and is a loosely bounded electron-hole pairing.
Organic photovoltaics aim to achieve moderate power conversion efficiencies at a low cost. The main drawback to OPVs is that they are much less efficient at converting light into electricity as compared to silicon based devices. However, OPVs are much less expensive than their silicon based counterparts. In addition, OPVs based on conjugated polymers can be fabricated by highly scaleable, high speed coating and printing processes, such as spin coating and ink-jet printing, to cover large areas on flexible substrates, enabling rapid mass-production. OPVs' low cost and manufacturing ease make them attractive even if their efficiencies are lower than that of existing technologies.
As a result, OPVs have emerged in recent years as promising alternatives to silicon based solar cells and a great deal of effort is being devoted, in both academic and industrial laboratories, to increase in power conversion efficiency and scale-up of the production processes. As previously mentioned, an attractive feature of OPVs based on conjugated polymers is that they can be fabricated by a coating process (e.g., spin coating or inkjet printing) to cover large areas on flexible substrates. The ability of OPVs to be fabricated by a coating process covering large flexible substrates was made possible by the discovery of photoinduced electron transfer from the excited state of a conjugated polymer (as the donor) onto fullerene (as the acceptor). Fullerene provides higher electron separation and collection efficiency compared to previously known electron acceptors.
Photovoltaic cells based on polymer/fullerene C60 planar heterojunctions have been previously reported. Blending a conjugated polymer and C60 (or its functionalized derivatives) results in moderate charge separation and collection efficiencies due to the formation of bulk donor-acceptor (D-A) heterojunctions. Much effort has gone into finding the best combination of D-A pairs and the optimum fabrication process.
The key to OPV technology is the mechanism of effective separation and transport of charge carriers, in absence of which energy is wasted. Energy conversion efficiency of OPVs has been approaching 5% under one sun irradiation using a conjugated polymer poly(3-hexyl-thiophene) (P3HT) as the electron donor and a fullerene derivative (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) as the electron acceptor. To achieve high performance, usually 50 wt % or more PCBM is required in the blend to create large numbers of exciton dissociation sites and to form an extensive percolation network for electron transport. PCBM is effective in bulk heterojunction solar cells because of its high solubility in organic solvents, such as toluene, and has better electron mobility as compared to C60. C60 on the other hand, is a stronger electron acceptor than PCBM and is more efficient in charge separation. In addition, PCBM is intrinsically more expensive than C60 because it involves the derivatization of C60 by complicated synthesis routes. The derivatization increases the overall cost of photovoltaic devices using PCBM as the electron acceptor.
Quantum Dots can be added to OPVs to form organic/inorganic hybrid photovoltaics. Quantum dots (QDs) are inorganic semiconductor crystals with a typical size of several nanometers. QDs possess properties that make them attractive for the development of high-efficiency, low-cost photovoltaics. For example, QDs can serve as electron acceptors when formed as a composite with a semiconducting polymer(s). Also, as compared to other electron acceptors (such as C60 in organic blend devices and TiO2 in dye-sensitized devices), QDs can absorb a large part of the solar spectrum and produce electron-hole pairs (excitons) that can be later dissociated and contribute to photogenerated current. However, inefficient transport of photogenerated charge carriers, like in OPVs, is a major source of efficiency loss in QD-polymer based photovoltaic devices.
Carbon nanotubes (CNTs), especially single wall carbon nanotubes (SWCNTs), are known as excellent electron transporters. Applications of CNTs in OPVs have been of much interest SWCNTs have in fact been employed as electrodes and blended with conjugated polymers to form bulk heterojunctions in the active layers. Kymakis et al. first reported a photovoltaic device based on the blend of SWCNTs and the conjugated polymer poly(3-octylthiophene) (P3OT). Adding SWCNTs to the P3OT matrix improved the photocurrent by more than two orders of magnitude. In a recent work, Pradhan et al. blended functionalized multi-walled carbon nanotubes (MWCNTs) into a P3HT polymer to provide extra dissociation sites and assist in charge transport in a P3HT-MWCNT/C60 double-layered device.
The major advantage of CNTs lies in their superior electron transport properties. However, nanotubes distributed within a polymer matrix are less efficient in separating photogenerated carriers than spherical C60 molecules that have a larger surface to volume ratio and it is difficult to disperse CNTs in a photoactive matrix. Purified CNTs blended with a polymer matrix have been found to be metastable and uniform distribution in a polymer matrix has been elusive.